dt-bindings: serial: add a new compatible string for SC9860

SC9860 use the same serial device which SC9836 uses, so added a new
compatible string to support SC9860 as well, also added an example
of how to describe this serial device in DT.
Signed-off-by: NChunyan Zhang <chunyan.zhang@spreadtrum.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/sprd-uart.txt

 * Spreadtrum serial UART
 
 Required properties:
-- compatible: must be "sprd,sc9836-uart"
+- compatible: must be one of:
+  * "sprd,sc9836-uart"
+  * "sprd,sc9860-uart", "sprd,sc9836-uart"
+
 - reg: offset and length of the register set for the device
 - interrupts: exactly one interrupt specifier
 - clocks: phandles to input clocks.
+
+Example:
+	uart0: serial@0 {
+		compatible = "sprd,sc9860-uart",
+			     "sprd,sc9836-uart";
+		reg = <0x0 0x100>;
+		interrupts = <GIC_SPI 2 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&ext_26m>;
+	};